Single-crystal diamond has been made use of, for example, in various fields related to tools such as cutting tools, acoustics, optics, electronic devices, and the like. This single-crystal diamond includes what is called IIa-type diamond and Ib-type diamond. The IIa-type diamond is high-purity diamond containing substantially no nitrogen representing an impurity, and the Ib-type diamond is an impurity-containing diamond containing nitrogen representing an impurity by approximately 0.1%. It has been known that IIa-type diamond is harder than Ib-type diamond based on comparison of hardness of these diamonds.
For example, H. Sumiya, Diamond & Related Materials 15 (2006) 1576-1579 (NPD 1) describes IIa-type diamond of which Knoop hardness varies with a temperature.
On the other hand, 12C and 13C have been known as carbon isotopes. For example, Japanese Patent Laying-Open No. 4-92894 (PTD 1) and Japanese Patent Laying-Open No. 4-92896 (PTD 2) describe that vapor-phase synthesized diamond excellent in thermal conductivity can be obtained by adjusting a content of this carbon isotope 12C or 13C and a content of nitrogen.